Fentanyl and fentanyl citrate are synthetic narcotic analgesics which have been confirmed being about 200 times more potent in analgesic activity than morphine in animal experiments. Nowadays fentanyl-containing reservoir-type and long-acting preparations for percutaneous absorption type are commercially available, for relieving pains due to cancer and said preparations can maintain the blood concentration of fentanyl practically at effective levels for 24 to 72 hours.
However, such reservoir-type and long-acting preparations for percutaneous absorption are disadvantageous in that the drug absorption after application thereof is fairly slow and the blood concentration arrives at an effective level only after 12 to 24 hours following the initial application, so that they cannot produce an immediate analgesic effect, in that because of their being reservoir-type preparations, they have the problem of fluid leakage, and in that they are very strong irritant to the lesion of application due to their containing ethanol.
Attempts have so far been made to produce matrix-type patches for percutaneous absorption as means for solving the above problems. For example, preparations for percutaneous absorption in which an acrylic adhesive is used are commercialized. However, the acrylic adhesive is generally inferior in drug-release, causing a problem: namely, a desired level of drug-release can be attained only by increasing the content of a main drug (Patent Document 1). The increase in the main drug content causes other problems, for example the problem of crystallization of the main drug during storage, and the problem of residual fentanyl in the preparation after application thereof.
On the other hand, while fentanyl-containing patches in which a styrene-isoprene-styrene block copolymer (hereinafter abbreviated as “SIS”) is used as a main base (SIS-based preparations) have also been disclosed in Patent Documents 2 and 3, there have not yet been developed any patches which can be stored for long term without causing crystallization and show stable skin adhesiveness and a sufficient main drug release at the application.    [Patent Document 1] Japanese Patent Publication (Tokuhyo) A 2004-524336    [Patent Document 2] WO 2003/070228    [Patent Document 3] Japanese Patent Publication A 2008-273865